Nice, It's pretty hard to make a tower defense game, and I like the old retro style. I don't like personally these types of games, but I can appreciate that you have done a very good job at this and put a lot of work into this. Can I ask you what type of movement you used for enemies? is it some pre-made behaviour or is it custom made? The enemies seem to move on a square path, which I could not do using the default path behaviour, So I thought I would ask.
And one more thing... I fail to see anything about separation...
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Thanks ! It's funny that you don't like this type of game, because I don't usually play them myself either. It wasn't before it was pointed out in the comments that the game looks like a tower defense that I realized it does indeed has elements of one.
As for the theme, there is a group of enemies that you have to separate or else they'll take your crystal and go away. You literally cannot win if you don't separate them, because individual monsters you place are only slightly stronger than a single hero. My main idea was to reverse the concept of "union makes strength".
For the enemies, I did use the default pathfinding behaviour, you can uncheck a box to disable diagonal movement. You can also define your pathfinding grid size, which was perfect for this game as it aligns destinations on the grid. It doesn't work perfectly, and honestly a lot of development time was spent fighting against common gdevelop pathfinding bugs and tweaking the level design to get the desired path behaviour.